Numerous applications exist for techniques which are capable of detecting and accurately measuring the presence of radiation and, more specifically, which have the facility for measuring and mapping radiation dose distributions in three dimensions with high spatial resolution. Such techniques find application in, for example, the assessment of radiation therapy techniques in the medical field, and in the detection and characterisation of potential radiation hazards in the nuclear and related industries.
In the context of radiation therapy, the use of aqueous gels containing Fricke dosimeter solution in combination with magnetic resonance imaging techniques has been suggested by Gore, J. C., et al., Phys. Med. Biol. 29:1189-1197; 1984, and further investigated by, inter alia, Schulz, R. J., et al., Phys. Med. Biol. 35:1611-1622; 1990, and Day, J. J. Phys. Med. 35:1605-1609; 1990, and the ability of this technique to map the dose distributions of the complex irradiations used in stereotactic radiosurgery have subsequently been demonstrated. However, such techniques suffer from a major disadvantage that is intrinsic to the Fricke gel medium, wherein ferric ions are able to diffuse quite freely through the gel after irradiation, thereby leading to a gradual blurring of the radiation dose pattern and a consequent loss of spatial resolution.
Several other dosimetry systems which employ gels or polymerisable resins are known from the prior art. Thus, U.S. Pat. No. 4,430,258 discloses a method of producing liquid equivalent solid gamma ray calibration standards which involves adding a first organic solvent to a calibrated aqueous solution of radioactive material to form a first solution; mixing the first solution with a polymerisable resin dissolved in a second organic solvent together with a hardening catalyst to form a second solution; and curing the second solution, whilst U.S. Pat. No. 4,588,698 discloses microencapsulation of solid phase scintillators in gels selectively permeable to diffusible radioactive label, these encapsulated scintillators being used to monitor the concentration of radioactive-tagged substances in fluid systems.
U.S. Pat. No. 4,350,607 discloses a radiation detector and dosimeter based on a finely-dispersed liquid suspended in a host liquid of high viscosity or gel. When radiation, and particularly neutron radiation of sufficient energy and intensity, comes into contact with such droplets, this can trigger volatilisation of the droplets, such that the volume of vapour evolved then serves as a measure of radiation intensity and dosage.
U.S. Pat. No. 4,779,000 discloses a direct reading, self-powered detector and dosimeter for gamma rays, and other low LET radiation, and optionally microwaves wherein a transparent elastic solid medium contains uniformly dispersed droplets of a very high vapour pressure detector liquid substantially immiscible with the elastic solid, the vaporisation of said droplets being sensitive to lightly ionizing radiation such as gamma rays and microwave radiation. Thus, the solid medium retains a record of each droplet vaporised, the number being proportional to the radiation dose.
U.S. Pat. No. 5,321,357 teaches a detection system wherein a visible and/or MRI visualisable permanent image is formed in a gel in a container which maintains the gel a dimensionally stable shape. A radiation polymerisable monomer is uniformly dispersed in the gel in storage stable form in a concentration effective to form an insoluble polymer in the gel which alters the relaxation time of the solvent phase of the gel in any area in which the polymer is formed. Polymerisation of the monomer is initiated in exposed areas by incident radiant energy, the resulting image being representative of the dose distribution of the radiant energy to which the gel is exposed. In a preferred embodiment, the gel contains a mixture of a linearly homopolymerisable monomer, e.g. a vinyl monomer, and a comonomer which is crosslinkably copolymerisable with the monomer, e.g. a monomer with two vinyl groups.
Optical scanning tomography is the subject of U.S. Pat. No. 6,218,673, wherein there is disclosed an optical scanner which provides three dimensional dosimetric data by scanning, with at least one light beam, a translucent medium exhibiting optical properties which change upon receipt of radiant energy representing a dose distribution of the energy. At least one detector is employed to gather data indicative of changes in the optical properties of the medium after scanning from multiple directions, thereby providing a representation of the optical properties in sections through the medium. Typical optical properties measured include optical density, light scattering, emitted light intensities, and combinations thereof, and the patent also describes methods of reconstructing a three-dimensional energy field as a series of two-dimensional images by applying an energy field to a translucent medium having distinct optical properties that change upon receipt of the energy field, optically scanning the translucent medium at various angles, detecting and measuring data indicative of optical changes in the medium, and preparing a two-dimensional image of the energy field by analyzing the changes in the optical properties.
WO-A-2004/079393 is concerned with a three-dimensional dosimeter for penetrating radiation, and describes a method of forming a three-dimensional dosimetric map in a solid translucent or transparent polymer and an article of manufacture comprising a polymer formulated to capture data imparted by incident penetrating radiation. The detects and displays a dose or doses of penetrating radiation by forming within the polymeric matrix a 3D dosimetric map which is measurable and quantifiable by known procedures. The dosimetric map is representative of the 3D distribution of the dose or doses of the penetrating radiation to which the polymer had been exposed and can be quantified at high spatial resolution, thereby providing an accurate, stable, storable record in three dimensions of the radiation exposure or dosing event(s).
Devices for the detection and measurement of radiation in industrial plants, such as the Radscan® 800 (available from BIL Solutions Ltd.), may be obtained commercially, and offer a convenient means for providing colour contour maps showing the spread and intensity of radiation over an area. Such devices are particularly useful when large areas require surveying, or in instances where access may be limited due to physical constraints or safety considerations, and they find particular application in, for example, the investigation of spillages of radioactive material, or the identification of y-hotspots during nuclear decommissioning operations. The Radscan® 800 comprises an optical arrangement which includes a CCD camera and laser range finder which operate in conjunction with caesium iodide detector attached to a photodiode and amplifier, these components being housed within a tungsten collimator.
The techniques of the prior art do, however, suffer from several disadvantages. For example, many systems—and particularly those associated with radiation therapy applications—demonstrate an inability to perform in high radiation backgrounds. Other common difficulties include practical problems in deployment, due to physical spatial constraints or the remoteness of locations in which investigations are to be performed. Furthermore, cost issues are often highly significant, with systems such as thee Radscan® 800 typically being expensive to purchase.
Thus, the present inventors have sought to address these issues and to provide a system and method for the detection and mapping of radiation which overcomes the difficulties associated with the prior art. The present invention, therefore, provides a means for the detection and mapping of radiation which performs effectively and efficiently in high radiation backgrounds, requires no electrical supply in order to function—and may, therefore, be deployed in a wide variety of locations and circumstances—and is relatively cheap and easy to manufacture.